In the conventional fourdrinier papermaking process, a water slurry, or suspension, of cellulosic fibers (known as the paper "stock") is fed onto the top of the upper run of an endless belt of woven wire and/or synthetic material that travels between two or more rollers. The belt, often referred to as a "forming fabric," provides a papermaking surface on the upper surface of its upper run which operates as a filter to separate the cellulosic fibers of the paper stock from the aqueous medium, thereby forming a wet paper web. The aqueous medium drains through mesh openings of the forming fabric, known as drainage holes, by gravity alone or with assistance from one or more suction boxes located on the lower surface (i.e., the "machine side") of the upper run of the fabric.
After leaving the forming section, the paper web is transferred to a press section of the paper machine, in which it is passed through the nips of one or more pairs of pressure rollers covered with another fabric, typically referred to as a "press felt." Pressure from the rollers removes additional moisture from the web; the moisture removal is often enhanced by the presence of a "batt" layer on the press felt. The paper is then conveyed to a drier section for further moisture removal. After drying, the paper is ready for secondary processing and packaging.
Press felts typically include two components: a base fabric and one or more batt layers. The base fabric is typically a woven construction that includes cabled or single monofilaments, plied multifilaments, or spun yarns. In a press felt, the base fabric may be a single layer fabric, an interwoven multilayer fabric, or a laminated construction comprising two or more distinct and separate fabric layers. The weave pattern(s) and yarn sizes and configurations employed in the base fabric are selected for the desired performance of the fabric; in particular, the fabric is designed for a desired balance of properties that include pressure uniformity, flow resistance, void volume, and compressibility.
The batt layer(s) of a fabric typically comprise staple fibers (usually synthetic fibers, such as nylon or polyester) that are applied in overlying layers to the base fabric. The thickness, denier and material of the batt fibers are typically selected for their contribution to the desired performance properties of the overall press felt.
In the typical manufacture of a press felt, batt fibers are "carded" to form a uniform web, then needled from this web into the base fabric. In the needling process, the batt web and base fabric are fed into a needle loom, where many needles (often on the order of 1,000-4,000 needles per lineal meter) are employed to insert the batt fibers into the base fabric. Conventionally, the needles are mounted in an industry standard "random" pattern on a needle board. The needle board is mounted on a needle beam, which in turn is mounted on the loom so that it can move in a reciprocating path in a direction normal to the batt web and fabric. Most commonly, the needles are of a "reverse-barb" configuration which snags batt fibers when moving in a direction toward the batt web and base fabric (ie., during insertion segment of the needle stroke) but fails to snag fibers when moving away from the batt and fabric (i.e., during the retraction segment of the needle stroke). Thus, as the needles are inserted through the batt and into the base fabric, the barbs of the needles engage the fibers of the batt web and thread them into the interstices of the base fabric. The needles can be retracted from the base fabric and batt web without the barbs snagging the batt fibers.
Generally, a press felt undergoes multiple passes through a needling loom, some of which may be conducted with different needle penetration depths, needle configurations, and fabric advance rates, and some of which simply involve the insertion of the needles into the base fabric without the addition of more batt fibers (when no additional batt fiber is applied, the needling typically serves to further engage batt fiber already present on the base fabric and reduce the thickness of the batt layer). Once needling is complete, the press felt is usually then subjected to some post-needling steps, such as heat setting, washing and singeing.
As noted above, during the needling process the needle board upon which the needles are mounted reciprocates along a path normal to the batt and fabric. The fabric and batt are advanced between needle strokes, either continuously (which is preferred for manufacturing efficiency) or intermittently, into position for subsequent needling.
Although continuous advancing of the batt and fabric is preferred for increased needling rate, this process can be deleterious for the finished press felt product, and in particular for the base fabric. The continuous motion of the base fabric can cause portions of the base fabric (especially the yarns of the base fabric that extend in a direction normal to the direction the fabric is moving) to contact and exert bending forces on the inserted needle. This interaction with the needle causes the yarns of the fabric to alternately stretch and compress in localized regions around each needle. Not only are the yarns of the base fabric stretched and compressed, the batt overlying these regions can become bunched or thinned. These heterogeneous regions of the press felt can adversely affect the smoothness and uniformity of the paper processed with the press felt. Also, the barbs of the needles can rub against the yarns of the fabric and have a "sawing" effect that may cut or weaken the yam. All of these effects can negatively impact the performance and consistency of the press felt during operation.